Methods for improving braze joints utilizing gold/copper/nickel brazing alloys

ABSTRACT

A method to improve the impact resistance of a braze joint between a tungsten/carbide/cobalt substrate and a substrate including titanium or alloy thereof includes utilizing a brazing material including gold, nickel, and copper present in respective amounts to improve the ductility of the braze joint.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of application Ser. No.11/930,858, filed Oct. 31, 2007, which is a Continuation-in-Part ofapplication Ser. No. 11/236,953, filed Sep. 28, 2005, now U.S. Pat. No.7,328,832, each of which is incorporated herein by reference in itsentirety.

TECHNICAL FIELD

Embodiments herein relate generally to methods for improving brazejoints, and more particularly to improving braze joints between titaniumalloy substrates and tungsten/carbide/cobalt (WC—Co) wear pads byutilizing gold/copper/nickel brazing alloys.

BACKGROUND OF THE INVENTION

In a gas turbine engine, air is pressurized in a compression moduleduring operation. The air channeled through the compression module ismixed with fuel in a combustor and ignited, generating hot combustiongases which flow through turbine stages that extract energy therefromfor powering the fan and compressor rotors and generating engine thrustto propel an aircraft in flight or to power a load, such as anelectrical generator.

The compression system includes a rotor assembly comprising a pluralityof rotor blades extending radially outward from a disk. Morespecifically, each rotor blade has a dovetail which engages with thedisk, a platform forming a part of the flow path, and an airfoilextending radially from the platform to a tip. The platform may be madeintegral to the blade or, alternatively, made separately.

In some designs, the rotor blade, especially those in a fan rotor andthe front stages of a multistage compression system, have a pair ofcircumferentially extending shrouds on the airfoil, one projecting fromthe pressure surface and one projecting from the suction surface. Theshrouds are located at a radial location between the blade dovetail andthe blade tip. In some other designs, the shrouds may be located at thetip of the blade airfoil. During normal operation of the compressionsystem, the blades twist and the shrouds on adjacent blades contact witheach other, forming a shroud ring that provides support to the blades.During engine operation, the shroud ring resists vibration and twistingof the blades. The term “midspan shroud” is used herein to refer to allsupports on fan and compression system blades that contact with eachother during operation, and includes all supports located anywhere onthe span of the blade, including supports at the tip of the blade. The“midspan shrouds” as used herein, may be located anywhere along theblade span, not just at the midpoint of the span.

During certain abnormal events, such as a bird impact, other foreignobject impact, or stalls during engine operation, the normal contactbetween the shrouds of adjacent blades is disturbed. The contact forcesbecome high and misaligned due to the impacts and the shrouds becomedisengaged fully or partially. This is called “shingling” of the blades.Shingling causes significant wear and tear damage on the midspanshrouds. When the speed of the compressor rotor drops, the shingledblades may rebound, causing further wear and tear on the shrouds.

Fan or compressor blades sometimes have wear pads brazed on the contactfaces of the midspan shrouds. Wear pads have been used on blades toaddress the wear problem. For example, some compressor blades contain abrazed-on WC—Co wear pad to reduce wear between two rubbing midspanshrouds.

The blades may comprise titanium or alloys thereof (i.e., Ti 6Al-4Vand/or Ti 8Al-1V-1 Mo alloys) having beta transus temperatures at orslightly above 1800° F. (about 982° C.). The wear pads areconventionally brazed to the titanium blade using atitanium-copper-nickel (TiCuNi) alloy braze foils. Diffusion occursbetween TiCuNi braze foil and WC—Co wear pad during high temperaturebraze. Titanium forms brittle compounds with the alloying elements ofthe wear pad in the braze joint. As a result, the braze joint provides ahigh hardness (about 1200 KHN) W—Co—Ti—Cu—Ni alloy. The braze interfaceexhibits cracking at impact energies as low as 0.30 Joules, and the wearpad may be liberated from the substrate at the brittle braze interfaceat an impact energy of about 0.60 Joules.

Industrially available braze alloys have been unable to meet the demandsfor high ductility and low cost necessary for aircraft engineapplications. Accordingly, there is a need for lower cost, highductility, impact resistant brazing alloys for brazing WC—Co substratesto titanium or titanium alloy substrates. In particular, there is a needfor brazing alloys for brazing WC—Co materials to titanium and titaniumalloys without forming a brittle braze interface.

BRIEF DESCRIPTION OF THE INVENTION

The above-mentioned need or needs may be met by exemplary embodimentswhich provide brazing materials that provide high ductility (i.e., lowhardness), sufficient wettability to the substrates, and can be brazedwithout harming the substrates.

An exemplary embodiment includes a method of improving the impactresistance of a braze joint between a midspan shroud of a fan orcompressor blade for a gas turbine engine and a wear pad brazed thereto.The method includes brazing the wear pad to the midspan shroud with abrazing material comprising about 40 to about 60 wt % gold, about 5 toabout 16 wt % nickel, and about 35 to about 55 wt % copper wherein thegold, nickel, and copper, are present in respective amounts to providethe brazing material with a post-braze hardness of between about 450 andabout 600 KHN, and the braze joint with an impact resistance of greaterthan about 0.60 Joules.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding part of thespecification. The invention, however, may be best understood byreference to the following description taken in conjunction with theaccompanying drawing figures in which:

FIG. 1 is a perspective view of an exemplary compressor blade having amidspan shroud.

FIG. 2 is a partial perspective view showing an exemplary midspanshroud-wear pad assembly.

FIG. 3 is a cross-sectional view of the midspan shroud-wear pad assemblytaken through 3-3 of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denotethe same elements throughout the various views, FIG. 1 shows anexemplary compressor blade 10 having an airfoil 11, dovetail 12, aplatform 14, and a midspan shroud 16 extending from the suction side 17of the airfoil 11. The midspan shroud 16 includes a contact face 18. Itwill be appreciated by those with skill in the art that a similarmidspan shroud extends from the pressure side of the airfoil. It willfurther be appreciated by those with skill in the art that blade 10 asshown is identified as a compressor blade, although the descriptions setforth herein are equally applicable to fan blades.

In fan and compressor rotor assemblies, the blades are arranged in thecircumferential direction around a disk. During engine operation, theblades airfoils twist, and the midspan shroud on the suction side of ablade comes into contact with the midspan shroud on the pressure side ofthe adjacent blade. The shrouds, when thus engaged with each other, forma stiff ring supporting the blades to prevent vibration. As the engineshuts down the shrouds disengage. As the engines operate through manycycles, the contact faces of the shrouds are subjected to significantwear and tear.

FIG. 2 illustrates an exemplary midspan shroud-wear pad assembly 20. Inthe assembly, a wear pad 22 is shown attached to contact face 18 of themidspan shroud 16. Wear pads are used with midspan shrouds 16 to addressthe wear problem addressed above. For example, some compressor bladesmade from titanium or alloys thereof may include a brazed-on WC—Co wearpad to prevent adhesive wear between two contacting midspan shrouds.

FIG. 3 illustrates a braze joint 24 at the interface between a midspanshroud 16 and wear pad 22. Braze joint 24 comprises a brazing material26 that, after brazing, provides an improvement in impact resistance ofthe braze joint 24 as compared to known prior art braze joints.

Embodiments disclosed here and are directed to brazing materials forbrazing a first substrate, such as a WC—Co wear pad 22, to a secondsubstrate, such as a midspan shroud comprising titanium or alloysthereof. Exemplary brazing materials include gold (about 40 to about 60wt %), nickel (about 5 to about 16 wt %), and copper (about 35 to about55 wt %).

In an exemplary embodiment, the substrates are brazed under brazingconditions to prevent damage to the mechanical properties of thesubstrates. In an exemplary embodiment, the brazing temperature isgenerally not greater than about 1800° F. (about 982° C.), which isgenerally below the beta transus temperatures of the titanium alloysubstrate. In exemplary embodiments, the brazing materials disclosedherein may include a nickel content sufficient to ensure wetting to bothWC—Co and titanium substrates, a copper content that is sufficientlyhigh to ensure ductility for impact resistance, and gold content that isreasonably low to ensure adequate cost. Further, the brazing materialsdisclosed herein are able to braze the first and second substrates usinga rapid induction heating process. In an exemplary embodiment, brazingoccurs at temperatures below about 1800° F. (about 982° C.) at brazetimes of from about 1 to about 10 minutes. In an exemplary embodiment,the braze time may be from about 1 minute to about 3 minutes. Theinduction heating may occur under vacuum of about 10⁻⁴ to about 10⁻⁵Torr. In an exemplary embodiment, the rapid braze process may allowbraze temperatures to be as high as about 1900° F. (about 1038° C.)without damaging the substrates.

In an exemplary embodiment, the weight percentages of gold, nickel, andcopper in the brazing material may be selected based upon the intendeduse of the brazing alloy. In particular, the weight percentages may beselected such that the resulting brazing alloy has a high impactresistance and high ductility (i.e., low hardness) after brazing andbrazing temperatures below the beta transus temperature of the substratesuch that the mechanical properties of the substrate are not negativelyaffected, for example, by way of phase transitions by high brazingtemperatures. It is contemplated that exemplary embodiments disclosedherein may be brazed at temperatures of up to about 1900° F. (1038° C.)without negatively impacting the substrates.

In an exemplary embodiment the brazing material may include about 40 toabout 60% by weight gold, about 5 to about 16% by weight nickel, andabout 35 to about 55% by weight copper.

In an exemplary embodiment, the brazing material may include from about45 to about 49% by weight gold, about 9 to about 11% by weight nickel,and about 35 to about 55% by weight copper.

In an exemplary embodiment, the brazing material may include from about40 to about 60% by weight gold, about 9 to about 16% by weight nickel,and about 40 to about 55% by weight copper.

In an exemplary embodiment, the brazing material may include from about45 to about 49 weight % gold, about 9 to about 11 weight % nickel andabout 41 to about 55 weight % copper.

In an exemplary embodiment, the brazing material may consist of about 47weight % gold, about 10 weight % nickel, and about 43 weight % copper.

In an exemplary embodiment, the gold, nickel, and copper are present inamounts such that the brazing material has a post-braze hardness ofbetween 450 and 600 KHN. In an exemplary embodiment, the post-brazehardness may be between about 550 to about 570 KHN. In an exemplaryembodiment, the copper content may be between about 40 and about 60weight %. In an exemplary embodiment, the copper content may be betweenabout 41 to about 55 weight %.

In an exemplary embodiment, nickel is present in an amount sufficient toprovide wetting to the first and second substrates during inductionheating. In an exemplary embodiment, the duration of the inductionheating process is at least about one minute. In an exemplaryembodiment, the induction heating process is not greater than about 10minutes. In an exemplary embodiment, the braze temperature is notgreater than about 1800° F. (982° C.). In an exemplary embodiment, thebraze temperature is between about 1750° F. (about 954° C.) to about1800° F. (982° C.). In other exemplary embodiments, the brazetemperature may be up to about 1900° F. (1038° C.). In other exemplaryembodiments, the braze temperature may be between about 1750° F. (about954° C.) to about 1900° F. (1038° C.).

The brazing materials disclosed herein may be provided in various forms.For example, the brazing materials may be provided as homogenouscompositions including gold, nickel, and copper. In other exemplaryembodiments, the brazing materials may be provided as powders. In otherexemplary embodiments, the brazing alloys may be provided as layered orlaminated films or foils.

In a powdered form, the brazing alloys may be provided as mixtures ofgold, nickel, and copper, and/or powders of alloys of one or more ofgold, nickel, and copper, wherein the metals are present in theappropriate quantities. In an exemplary embodiment, the powders may forma homogeneous alloy upon being heated to the appropriate brazingtemperature. For example, an exemplary brazing material may be providedas a dispersion of copper powder, gold/copper/nickel powder, gold/nickelpowder or mixtures thereof as appropriate.

In a layered form, the gold, nickel, copper may be provided in separatelayers, thereby providing homogeneous alloys upon being heated to theappropriate brazing temperature. For example, a brazing alloy may beprovided as a laminated film or a layered material. In an exemplaryembodiment, the brazing material may comprise a layer of copper foilpositioned between layers of gold/nickel foil. Any combination of layersmay be utilized to provide the alloying metals in the appropriatequantities. In other exemplary embodiments, the brazing material may beprovided as layered or laminated films or foils.

In the layered form, the metals or alloys of the gold/nickel/copperbrazing material may be provided in separate layers that provide asubstantially homogeneous alloy during the brazing process. Thoseskilled in the art will appreciate that various arrangements and numbersof layers and various combinations of metals and/or alloys are withinthe scope of this disclosure. The layered material may be used in a flat(i.e., planar) configuration, or may be rolled or otherwise shaped priorto brazing.

An exemplary embodiment includes a brazing assembly comprising atungsten/carbide/cobalt (WC—Co) substrate, a titanium or titanium alloysubstrate, and a brazing material disposed between the substrates. Anexemplary brazing material includes 40 to about 60 percent by weightgold, about 5 to about 16 percent by weight nickel, and about 35 toabout 55 percent by weight copper.

An exemplary embodiment comprises an article including a first substratecomprising tungsten/carbide/cobalt material, a second substratecomprising titanium or alloys thereof, and a braze joint at theinterface of the first substrate and the second substrate. The brazejoint is formed from an exemplary brazing material including about 40 toabout 60 percent by weight gold, about 5 to about 16 percent by weightnickel, about 35 to about 55 percent by weight copper.

Example 1

A brazing material is prepared by positioning a copper foil between twolayers of gold/nickel braze foil. The thickness of each layer wasselected such that the resulting material included about 47 wt % gold,about 10 wt % nickel, and about 43 wt % copper with respect to the totalweight of the layered material. The resulting material had a brazingtemperature of about 1775° F. (about 968° C.).

Example 2

A brazing material was prepared by positioning a copper foil between twolayers of gold/nickel braze foil. The thickness of each layer wasselected such that the resulting material included about 53 wt % gold,about 11 wt % nickel and about 36 wt % copper with respect to the totalweigh of the layered material. The resulting layered material has abrazing temperature of about 1795° F. (about 979° C.).

Example 3

The brazing material of Example 1 was rolled up and positioned between aWC—Co (2-10% cobalt) wear pad and a titanium alloy (90 wt % Ti, 6 wt %Al and 4 wt % V) midspan shroud and the assembly was raised to atemperature of about 1800° F. (by way of induction heating) for about 10minutes under vacuum (about 10⁻⁴ Torr). After the assembly was allowedto cool, the braze joint was determined to have a hardness of about 550KHN.

Example 4

The brazing material of Example 2 was rolled up and positioned between aWC—Co (2-10% cobalt) wear pad and a titanium alloy (90 wt % Ti, 6 wt %Al and 4 wt % V) midspan shroud and the assembly was raised to atemperature of about 1800° F. (by way of induction heating) for about 10minutes under vacuum (about 10⁻⁴ Torr). After the assembly was allowedto cool, the braze joint was determined to have a hardness of about 570KHN.

Example 5

An exemplary gold/nickel/copper brazing material as disclosed herein wasemployed to braze a WC—Co wear pad to the midspan shroud (e.g., 16) ofan airfoil (e.g., 11). The braze temperature reached about 1885° F.(about 1029° C.) at some locations. A cut-up inspection of the airfoilshowed no damage from the brazing process to the body of the airfoil.

Accordingly, the gold/nickel/copper brazing alloys disclosed herein areductile and impact resistant with respect to titanium/nickel/copperbrazing alloys and exhibit excellent wetting when used to join variousWC—Co material to various titanium alloys.

In an exemplary embodiment, a method for improving the impact resistanceof a braze joint 24 between a first substrate (i.e., wear pad 22) and asecond substrate. (i.e., midspan shroud 16) includes providing a brazingassembly including the first substrate the second substrate and abrazing material disposed therebetween. Any of the aforementionedexemplary brazing materials 26 may be utilized. In particular, the wearpad 22 may be brazed to the contact surface 18 of the midspan shroud. Inan exemplary embodiment, the brazing material comprises gold, nickel,and copper in respective amounts such that, after brazing, the brazejoint 24 has a hardness of between about 450 and 600 KHN and an impactresistance of greater than about 0.60 Joules. The brazing material maybe provided in any of the aforementioned forms, including layered,powdered, or homogeneous forms.

In an exemplary method, providing a midspan shroud includes providing amidspan shroud requiring repair due to a damaged wear pad. In anexemplary method, the damaged wear pad is removed by mechanical,chemical or a combination of mechanical and chemical methods. Forexample, the midspan shroud of a blade may be subjected to a grindingprocess to remove the worn wear pad and prior braze material.Alternately, the worn wear pad may be chemically removed. If necessary,the midspan shroud may be subjected to further processes in preparationfor brazing on a new wear pad (i.e., material build up, machining tospecifications, and the like).

An exemplary method further includes brazing the first substrate to thesecond substrate. In an exemplary method, an induction heating processis utilized for brazing. The brazing assembly (i.e., midspan shroud 16,brazing material 26, wear pad 22) may be placed into a vacuum chamber.The midspan shroud 16 may be placed into an induction coil. AC currentpassing through the coil generates a magnetic field in the midspanshroud, generating eddy currents in the shroud to rapidly increase thetemperature to the brazing range. In exemplary embodiments, the brazingtemperature is between about 1750° F. (about 954° C.) to about 1800° F.(982° C.). Duration for the braze may be about 1-3 minutes. In otherexemplary embodiments, the braze temperature may be as high as about1900° F. (about 1038° C.). An exemplary braze duration may be up toabout 10 minutes. In an exemplary embodiment, the braze duration is atleast about 1 minute. In an exemplary embodiment, the brazing isaccomplished under vacuum of about 10⁻⁴ to about 10⁻⁵ Torr.

With reference to FIGS. 1-3, an exemplary embodiment comprises anarticle including at least one braze joint. The article may be a fan orcompressor blade 10. The blade 10 may include a midspan shroud-wear padassembly 20. The wear pad 22 is attached to contact face 18 of themidspan shroud through the braze joint 24. Braze joint 24 is formed fromany of the exemplary brazing materials disclosed herein. The braze joint24 exhibits increased ductility (decreased hardness) and thus providesimproved impact resistance.

Exemplary embodiments include a braze assembly illustrated in FIG. 3. Anexemplary braze assembly includes a first substrate, such as wear pad 22which may comprise WC—Co. The exemplary braze assembly includes brazingmaterial 26 disposed between the first and second substrates. Anexemplary brazing material comprises about 40 to about 60 percent byweight gold, about 5 to about 16 percent by weight nickel, and about 35to about 55 percent by weight copper. Other exemplary brazing materialsdisclosed here may be used in the braze assembly.

Thus, embodiments disclosed herein provide braze assemblies, brazingmaterials, methods of improving the impact resistance of a brazed joint,and brazed articles. The brazing materials disclosed herein providepost-braze joints having increased ductility. The gold/nickel/copperbrazing materials disclosed herein provide good wetting when brazingWC—Co substrates to titanium or alloys thereof under brazing conditionsto minimize or prevent damage to the substrates. For example, thebrazing temperature may be less than about 1800° F. (982° C.) duringinduction heating of less than about 10 minutes. In other exemplaryembodiments, rapid induction heating may allow for increased brazetemperatures of up to about 1900° F. (about 1038° C.).

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral languages of the claims.

1. A method of improving the impact resistance of a braze joint betweena midspan shroud of a fan or compressor blade for a gas turbine engineand a wear pad brazed thereto, the method comprising: brazing the wearpad to the midspan shroud with a brazing material including: about 40 toabout 60 wt % gold; about 5 to about 16 wt % nickel; about 35 to about55 wt % copper; wherein the gold, nickel, and copper are present inrespective amounts to provide the brazing material with a post-brazehardness of between about 450 and about 600 KHN, and the braze jointwith an impact resistance of greater than about 0.60 Joules.
 2. Themethod according to claim 1 further comprising: providing the midspanshroud, wherein the midspan shroud includes a face for receiving thewear pad; providing the wear pad; and disposing the brazing materialbetween the wear pad and the face to provide a brazing assembly.
 3. Themethod according to claim 2 wherein the brazing material is in a formselected from a homogeneous alloy form, a powder form, or a layeredform.
 4. The method according to claim 3 wherein the brazing material isin the layered form, and wherein the method includes: positioning alayer consisting essentially of a copper foil between a layer ofnickel-containing foil and a layer of gold-containing foil.
 5. Themethod according to claim 2 wherein providing the midspan shroudincludes: providing a midspan shroud requiring repair due to a damagedwear pad; and removing the damaged wear pad.
 6. The method according toclaim 5 wherein the damaged wear pad is removed chemically,mechanically, or a combination of chemically and mechanically.
 7. Themethod according to claim 2 further comprising: subjecting the brazingassembly to an induction heating process for a duration of at leastabout 1 minute and less than 10 minutes at braze temperatures of lessthan about 1900° F. (about 1038° C.).